Radiant or electromagnetic energy emitters and distributors find a wide range of applications in modern society. Visible illumination systems, for example, illuminate areas and surfaces to enable use by personnel even though natural ambient lighting might be insufficient. Infrared illumination is a critical component of many night-vision technologies.
Different applications of radiant energy illumination systems require different performance characteristics. For example, a visible illumination application might require that the lighting system provides a desired minimum intensity over a flat surface of specified dimensions about an axis of the lighting system, at a known distance from the system along its axis. Simple radiation sources, such as light emitting diodes (LEDs) or light bulbs with reflectors and/or lenses typically provide a high intensity radiation in regions close to the axis, but the intensity drops off quickly at angles approaching the horizon. On an illuminated surface, the intensity is not uniform, as often desired.
To provide a desired illumination at edges of a design footprint, the source often will emit substantially higher amounts of radiation than necessary along the axis. Although such an approach may meet minimum requirements, it requires an excessive amount of power.
Prior attempts to provide desired intensity distributions have involved complex arrangements of sources, lenses and reflectors. These complex arrangements tend to be relatively expensive and sensitive to problems of misalignment, which limits ruggedness and durability.
As an example of a difficult lighting application, consider illumination of the vehicle filling area, under a canopy, in a modern self-serve type gas station or the like. Light fixtures are attached to the underside of the canopy. Such fixtures must distribute light downward toward the ground. The illumination system must also provide some illumination at an angle to illuminate vertical surfaces of the pumps. To achieve desired lighting, the requirements for such an application actually specify desired downward intensity and angular coverage and require some amount of up-lighting at an angle onto the underside of the canopy. A problem arises, however, where illumination systems meeting such requirements also distribute a substantial portion of the light outward at angles approaching the horizon. The light emitted in this later direction actually "trespasses" on adjacent properties, and in many instances, is disturbing to persons living on or using the adjacent properties.
U.S. Pat. No. 5,733,028 issued Mar. 31, 1998 to Ramer et al. discloses a number of embodiments of illumination systems that utilize constructive occlusion. With this technology, a mask occludes an active optical surface, typically a Lambertian surface formed by the aperture of a diffusely reflective cavity. In most embodiments, a reflective shoulder surrounds all or at least a portion of the aperture. The mask, cavity and shoulder distribute radiant energy from within the cavity out over an area, with a tailored intensity distribution. The disclosure there emphasizes uniformity of the intensity distribution, for example with respect to angles extending over a hemispherical radiation pattern. Adjustment of the parameters of the constructive occlusion system enables the system designer to tailor the system performance to a wide range of applications.
However, a need still exists for radiant energy or electromagnetic emission and distribution systems, which can satisfy certain specialized requirements as to a desired intensity distribution. Such systems must be relatively simple in structure, to minimize cost and maximize durability. Also, such systems should be able to achieve a desired intensity distribution including at least some area beyond the horizon of the aperture, and in some instances exhibiting a dead zone at or near the horizon. In the gas station example, such a dead zone at or near the horizon would help to minimize light trespass on adjacent properties.